Process for liquefying natural gas and producing hydrocarbons

ABSTRACT

A process for the preparation of liquid hydrocarbons from a light hydrocarbonaceous feedstock in combination with a process for liquefying natural gas, which liquefaction process involves the steps of
     (a) passing the natural gas at liquefaction pressure through the product side of a main heat exchanger;   (b) introducing cooled liquefied refrigerant at refrigerant pressure in the cold side of the main heat exchanger, allowing the cooled refrigerant to evaporate at the refrigerant pressure in the cold side of the main heat exchanger to obtain vaporous refrigerant at refrigerant pressure, and removing vaporous refrigerant from the cold side of the main heat exchanger;   (c) removing the liquefied gas at liquefaction pressure from the product side of the main heat exchanger;   (d) allowing the cooled liquefied gas to expand to a lower pressure to obtain expanded fluid;   (e) supplying the expanded fluid to a separator vessel;   (f) withdrawing from the bottom of the separator vessel a liquid product stream;   (g) withdrawing from the top of the separator vessel a gaseous stream;   (h) introducing the gaseous stream obtained in step (g) as feed and/or fuel in the process for the preparation of liquid hydrocarbons,
 
which process for the preparation of hydrocarbons involves converting a light hydrocarbonaceous feedstock into synthesis gas, followed by catalytic conversion of the synthesis gas into liquid hydrocarbons.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of liquidhydrocarbons from a light hydrocarbonaceous feedstock in combinationwith a process for liquefying natural gas.

BACKGROUND OF THE INVENTION

Processes for the liquefaction of natural gas are well known. In thisrespect reference is made to for example GB 1,572,899, U.S. Pat. No.4,504,296, 4,545,795, 4,456,459, 3,203,191, EP 834,046 and WO 97/32172.Processes for the preparation of liquid hydrocarbons from lighthydrocarbonaceous feedstock are also well known. In this respectreference is made to WO 98/11037, EP 583,837, EP 497,425, WO 94/21512,WO 97/12118, WO 91/15446, U.S. Pat. No. 4,833,170, WO 99/15483 and EP861,122.

An integrated process and plant for the production of liquidhydrocarbons from light hydrocarbonaceous feedstock, the liquefaction ofnatural gas and the production of electrical power has been described inWO 98/36038. However, in this process there is no integration betweenthe LNG unit and the synfuel unit.

SUMMARY OF THE INVENTION

The present process, is directed to an integrated process for thepreparation of liquefied natural gas and the preparation of liquidhydrocarbons from a light hydrocarbonaceous product. The integrationresults in a much larger production of liquefied natural gas for a givenrefrigeration compression power utilization. Such a production increaseis not possible in the scheme described in WO 98/36038. The increasedamount of liquefied natural gas is obtained by increasing the amount ofend flash gas made in the liquefaction process and using this end flashgas as feed for the production of liquid hydrocarbons by means ofconversion of the end flash gas into synthesis gas and catalyticconversion of the synthesis gas into liquid hydrocarbons (the FischerTropsch synthesis process) and/or using the end flash gas in theproduction of hydrogen and/or synthesis gas having a very highhydrogen/carbon monoxide ratio, and using this hydrogen or synthesis gasin the catalytic conversion of synthesis gas into liquid hydrocarbons.Further, an improved energy/power integration is obtained, especially bythe use of off gasses, for instance off gasses from the hydrocarbonsynthesis process, but also other hydrogen and/or hydrocarbon gassesstreams may be used, for the production of power needed in the naturalgas liquefaction plant.

The present process, therefore, is directed to a process comprising:

(a) passing the natural gas at liquefaction pressure through a main heatexchanger having a product side and a cold side, wherein the natural gasenters the heat exchanger through the product side;

(b) introducing cooled liquefied refrigerant at refrigerant pressure inthe cold side of the main heat exchanger, allowing the cooledrefrigerant to evaporate at the refrigerant pressure in the cold side ofthe main heat exchanger to obtain vaporous refrigerant at refrigerantpressure, and removing vaporous refrigerant from the cold side of themain heat exchanger;

(c) removing the liquefied gas at liquefaction pressure from the productside of the main heat exchanger;

(d) allowing the cooled liquefied gas to expand to a lower pressure toobtain expanded fluid;

(e) supplying the expanded fluid to a separator vessel having a top anda bottom;

(f) withdrawing from the bottom of the separator vessel a liquid productstream;

(g) withdrawing from the top of the separator vessel a gaseous stream;and

(h) introducing the gaseous stream obtained in step (g) as feed and/orfuel in the process for the preparation of liquid hydrocarbons, whichprocess for the preparation of hydrocarbons involves converting a lighthydrocarbonaceous feedstock into synthesis gas, followed by catalyticconversion of the synthesis gas into liquid hydrocarbons.

As indicated above, the advantage of the process of the presentinvention results in an increased liquefaction efficiency and hence toan incremental liquefied natural gas production for a givenrefrigeration compression power utilization. This increase in productionmay be up by 50% when compared with the production with minimum end gasflash production. Further, a considerable amount of energy saving may beobtained by using off gasses from the hydrocarbon manufacture (paraffinsynthesis, hydrocracking, hydrotreatment etc.) for the generation ofenergy to be used in the liquefaction plant. An additional advantage isthat in those cases in which the natural gas comprises (substantial)amounts of nitrogen or other light compounds as helium or neon, thisnitrogen and/or light compounds will be removed from the liquefiednatural gas in the end gas flash separation step.

In the liquefaction of natural gas, the liquefied natural gas is in thefirst instance obtained at a relatively high pressure, usually between4.0 to 7.0 MPa. For storage and/or shipping the pressure of the naturalgas is usually decreased to atmospheric pressure or a pressure slightlyabove atmospheric pressure, e.g. 0.10 to 0.15 MPa. Such a pressurereduction is often called an end flash reduction, resulting in end flashgas (sometimes also called “flash gas”) and liquefied natural gas. Anadvantage of such an end flash reduction is that optional boilingcomponents (e.g. nitrogen, neon and/or helium) are, at least partly,removed from the liquefied natural gas. The end flash gas is usuallyused as fuel gas, preferably for operation of the equipment of theliquefaction plant.

DETAILED DESCRIPTION OF THE INVENTION

A minimum-production of end flash gas is obtained when the liquefiednatural gas is obtained by temperature of −161° C. at a pressure usuallybetween 1.5 and 7.0 MPa. Typically natural gas is liquefied (i.e.condensed) at a pressure between 3.5 and 7.0 MPa and it may be furthersubcooled to around −161° C. at pressures between 1.5 and 7.0 Mpa. It ispossible to cool the liquefied natural gas to a higher temperature, e.g.−150° C., or even up to −120° C., which results in a lower specificcooling duty, i.e. for a given refrigeration capacity more liquefiednatural gas can be obtained. Upon pressure letdown to atmosphericpressure vapor will be generated. This pressure letdown may be carriedout in several stages, e.g. one or two stages, the last one beingoperated at a pressure similar or close to the pressure of the liquefiednatural gas storage tanks. A one vessel system is preferred. The vaporgenerated in the vessel is called end flash gas. The end flash gas andthe liquefied gas are in equilibrium and both have the same temperature(−161° C.). The end gas flash may be used for the generation of energyor may be used in another useful disposal route, usually after acompression step. Prior to compression or other use, the cold containedin the end flash gas is usually recovered, e.g. by the refrigerantfluid, thereby enhancing the liquefaction efficiency further. Theproduction of end flash gas is preferably heat integrated with the mainheat exchanger, preferably in a cold box.

As indicated above, it has appeared very beneficial to use the end flashgas in the production of liquid hydrocarbons.

The end flash gas is mainly methane, but it may contain some lightcomponents, especially nitrogen, helium and/or neon.

In a preferred way the amount of end flash gas is between 5 and 35% ofthe amount of LNG produced, preferably between 10 and 30%, morepreferably between 15 and 25%. The pressure at which the liquefiednatural gas is made is usually between 1.5 and 7.0 MPa, preferablybetween 5.0 and 7.0 MPa. The temperature of the liquefied natural gasafter the heat exchange is usually between −155 and −125° C., preferablybetween −150 and −130° C. The feed for the liquefied gas process maycontain typically up to 15 vol % nitrogen based on total stream, usuallybetween 0.1 and 12 vol %, often between 0.2 and 10 vol %.

The main heat exchanger may comprises any typical heat exchanger used inthe LNG industry. It may comprise one large, single heat exchanger, orit may comprise two or more, e.g. three heat exchangers often groupedtogether in the one so-called cold box. It comprises for instance theheat exchangers used in mixed refrigerant cycle LNG plants, in cascadecycle LNG plants, or in expander cycle LNG plants, including variationson these cycles, as (propane) precooling. In this respect reference ismade to Kirk Othmer, Enc. of Chem. Techn., fourth edition, Volume 7,p663-668.

In a preferred embodiment inerts, especially nitrogen, are removed fromthe end flash gas, for instance in a stripper or distillation unit. Thisunit is preferably heat integrated with the main heat exchanger,preferably in one cold box. This will improve the efficiency of thepreparation of liquid hydrocarbons.

In another preferred embodiment the pressure relief of the liquidnatural gas is carried out by means of an expander.

Several methods may be used for the conversion of end flash gas intosynthesis gas. Preferred are (catalytic) partial oxidation, steammethane reforming, or (integrated) combinations of these two processes.Usually the end flash gas will be mixed with a light hydrocarbonaceousstream. This hydrocarbonaceous feed suitably is methane, natural gas,associated gas or a mixture of C₁₋₄ hydrocarbons. The feed comprisesmainly, i.e. more than 90 v/v %, especially more than 94%, C₁₋₄hydrocarbons, especially comprises at least 60 v/v percent methane,preferably at least 7.5 percent, more preferably 90 percent. Verysuitably natural gas or associated gas is used. Suitably, any sulphur inthe feedstock is removed.

The partial oxidation of the end flash gas/light hydrocarbonaceous feedstream, producing a mixture of carbon monoxide and hydrogen, may takeplace in an oxidation unit according to various established processes.These processes include the Shell Gasification Process. A comprehensivesurvey of this process can found in the Oil and Gas Journal, Sep. 6,1971, pp. 86-90.

The oxygen containing gas may by air (containing about 21 percentoxygen), or oxygen enriched air, suitably containing up to 100 percentof oxygen, preferably containing at least 60 volume percent oxygen, morepreferably at least 80 volume percent, more preferably at least 98volume percent of oxygen. Oxygen enriched air may be produced viacryogenic techniques, but may also be produced by a membrane basedprocess, e.g. the process as described in WO 93/06041.

To adjust the H₂/CO ratio in the syngas, carbon dioxide and/or steam maybe introduced into the partial oxidation process. Preferably up to 15%volume based on the amount of syngas, preferably up to 8% volume, morepreferably up to 4% volume, of either carbon dioxide or steam is addedto the feed. As a suitable steam source, water produced in thehydrocarbon synthesis may be used. As a suitable carbon dioxide source,carbon dioxide from effluent gasses may be used. The H₂/CO ratio of thesyngas is suitably between 1.5 and 2.3, preferably between 1.8 and 2.1.

If desired, additional amounts of hydrogen and/or synthesis gas having ahigh hydrogen/carbon monoxide ratio may be made by steam methanereforming, preferably in combination with the water shift reaction. Anycarbon monoxide and carbon dioxide produced together with the hydrogenmay be used in the hydrocarbon synthesis reaction or recycled toincrease the carbon efficiency.

The percentage of end flash gas/light hydro-carbonaceous feed streamwhich is converted in the first step of the process of the invention issuitably 50-99% by weight and preferably 80-98% by weight, morepreferably 85-96% by weight.

The gaseous mixture, comprising predominantly hydrogen carbon monoxideand optionally nitrogen, is contacted with a suitable catalyst in thecatalytic conversion stage, in which the normally liquid hydrocarbonsare formed. Suitably at least 70 v/v % of the syngas is contacted withthe catalyst, preferably at least 80%, more preferably at least 90%,still more preferably all the syngas. The conversion may be carried outin one or more stages.

The (normally liquid) hydrocarbons produced in the hydrocarbon synthesisprocess of the invention are suitably C₃₋₂₀₀ hydrocarbons, more suitablyC₄₋₁₂₀ hydrocarbons, especially C₅₋₈₀ hydrocarbons, more especially,after hydrocracking, C₆₋₂₀ hydrocarbons, or mixtures thereof. Thesehydrocarbons or mixtures thereof are liquid at temperatures between 5and 30° C. (1 bar), especially at 20° C. (1 bar), and usually areparaffinic of nature, while up to 20 wt %, preferably up to 5 wt %, ofeither olefins or oxygenated compounds may be present.

The catalysts used in the conversion unit for the catalytic conversionof the mixture comprising hydrogen and carbon monoxide into hydrocarbonsare known in the art and are usually referred to as Fischer-Tropschcatalysts. Catalysts for use in the Fischer-Tropsch hydrocarbonsynthesis process frequently comprise, as the catalytically activecomponent, a metal from Group VIII of the Periodic Table of Elements.Particular catalytically active metals include ruthenium, iron, cobaltand nickel. Cobalt is a preferred catalytically active metal.

The catalytically active metal is preferably supported on a porouscarrier. The porous carrier may be selected from any of the suitablerefractory metal oxides or silicates or combinations thereof known inthe art. Particular examples of preferred porous carriers includesilica, alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica and titania.

The amount of catalytically active metal on the carrier is preferably inthe range of from 3 to 300 pbw per 100 pbw of carrier material, morepreferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the catalyst may also comprise one or more metals or metaloxides as promoters. Suitable metal oxide promoters may be selected fromGroups IIA, IIIB, IVB, VB and VIB of the Periodic Table of Elements, orthe actinides and lanthanides. In particular, oxides of magnesium,calcium, strontium, barium, scandium, yttrium, lanthanum, cerium,titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium andmanganese are most suitable promoters. Particularly preferred metaloxide promoters for the catalyst are manganese and zirconium oxide.Suitable metal promoters may be selected from Groups VIIB or VIII of thePeriodic Table. Rhenium and Group VIII noble metals are particularlysuitable, with platinum and palladium being especially preferred. Theamount of promoter present in the catalyst is suitably in the range offrom 0.01 to 100 pbw, preferably 0.1 to 40, more preferably 1 to 20 pbw,per 100 pbw of carrier.

The catalytically active metal and the promoter, if present, may bedeposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination at a temperature of generally from350 to 750° C., preferably a temperature in the range of from 450 to550° C. The effect of the calcination treatment is to remove crystalwater, to decompose volatile decomposition products and to convertorganic and inorganic compounds to their respective oxides. Aftercalcination, the resulting catalyst may be activated by contacting thecatalyst with hydrogen or a hydrogen-containing gas, typically attemperatures of about 200 to 350° C.

The catalytic conversion process may be performed in the conversion unitunder conventional synthesis conditions known in the art. Typically, thecatalytic conversion may be effected at a temperature in the range offrom 100 to 600° C., preferably from 150 to 350° C., more preferablyfrom 180 to 270° C. Typical total pressures for the catalytic conversionprocess are in the range of from 1 to 200 bar absolute, more preferablyfrom 10 to 70 bar absolute. In the catalytic conversion process mainly(at least 70 wt %, preferably 80 wt %) C₅+ hydrocarbons are formed.

Preferably, a Fischer-Tropsch catalyst is used, which yields substantialquantities of paraffins, more preferably substantially unbranchedparaffins. A part may boil above the boiling point range of theso-called middle distillates. A most suitable catalyst for this purposeis a cobalt-containing Fischer-Tropsch catalyst. The term “middledistillates”, as used herein, is a reference to hydrocarbon mixtures ofwhich the boiling point range corresponds substantially to that ofkerosene and gas oil fractions obtained in a conventional atmosphericdistillation of crude mineral oil. The boiling point range of middledistillates generally lies within the range of about 150 to about 360°C.

The higher boiling range paraffinic hydrocarbons, if present, may beisolated and subjected in an optional hydrocracking unit to a catalytichydrocracking which is known per se in the art, to yield the desiredmiddle distillates. The catalytic hydrocracking is carried out bycontacting the paraffinic hydrocarbons at elevated temperature andpressure and in the presence of hydrogen with a catalyst containing oneor more metals having hydrogenation activity, and supported on acarrier. Suitable hydrocracking catalysts include catalysts comprisingmetals selected from Groups VIB and VIII of the Periodic Table ofElements. Preferably, the hydrocracking catalysts contain one or morenoble metals from group VIII. Preferred noble metals are platinum,palladium, rhodium, ruthenium, iridium and osmium. Most preferredcatalysts for use in the hydro-cracking stage are those comprisingplatinum.

The amount of catalytically active metal present in the hydrocrackingcatalyst may vary within wide limits and is typically in the range offrom about 0.05 to about 5 parts by weight per 100 parts by weight ofthe carrier material.

Suitable conditions for the optional catalytic hydrocracking in ahydrocracking unit are known in the art. Typically, the hydrocracking iseffected at a temperature in the range of from about 175 to 400° C.Typical hydrogen partial pressures applied in the hydrocracking processare in the range of from 10 to 250 bar.

In a further preferred process, the off gas of the Fischer Tropschreaction may be used for the generation of energy for the liquefactionprocess. Preferably at least part of the off gas is used, especially atleast 60%, preferably at least 90% is used. In addition, also off gas ofany hydrocracking operation may be used for the generation of power tobe used in the liquefaction process.

A still further improvement is obtained by using the energy obtained inthe catalytic oxidation process for the generation of synthesis gasand/or the Fischer Tropsch reaction for the liquefaction of natural gas.Both processes are highly exothermic, and usually the excess heat isremoved by steam cooling. The steam thus generated may be used for theproduction of power to be used in the liquefaction process. Part of theenergy may also be used in the hydrocarbon synthesis plant, butcertainly the excess may be used in the liquefaction process.

In the case that only a limited amount of end flash gas would beavailable, the preference is given to use it in the manufacturing (steammethane reforming) of hydrogen and/or synthesis gas having a highhydrogen/carbon monoxide ratio. It is preferred to use the end flash gasfirst as feed for the steam methane reforming process, and, whensufficient gas is available, also as fuel for the process, usually toreplace off gas from the hydrocarbon synthesis reaction. When more endflash gas is available, or in the case that it is not desired to use itas fuel for the steam methane reforming process, this can be used in thepreparation of synthesis gas for the hydrocarbon synthesis reaction.

For a liquefaction plant designed to produce 4 mpta LNG the amount ofend flash gas could be between 0.4 and 1.2 mpta. Such an amount of endflash gas may result in an increase of LNG production between 20 to 50%extra LNG. Export to a hydrocarbon synthesis plant of sufficient size,using a partial oxidation reaction for the production of synthesis gas,will be sufficient for the required hydrogen production of such a plantas well as to partially replace the feed for the partial oxidationreaction. The energy generated by the catalytic partial oxidation andthe hydrocarbon synthesis reaction (steam and off gas conversion) willbe sufficient to supply the required power to the liquefaction plant andthe hydrocarbon synthesis plant.

1. A process for the preparation of liquid hydrocarbons from a lighthydrocarbonaceous feedstock in combination with a process for liquefyingnatural gas, which liquefaction process comprises the steps of: (a)passing the natural gas at liquefaction pressure through a main heatexchanger having a product side and a cold side, wherein the natural gasenters the heat exchanger through the product side; (b) introducingcooled liquefied refrigerant at refrigerant pressure in the cold side ofthe main heat exchanger, allowing the cooled refrigerant to evaporate atthe refrigerant pressure in the cold side of the main heat exchanger toobtain vaporous refrigerant at refrigerant pressure, and removingvaporous refrigerant from the cold side of the main heat exchanger; (c)removing the liquefied gas at liquefaction pressure from the productside of the main heat exchanger; (d) allowing the cooled liquefied gasto expand to a lower pressure to obtain expanded fluid; (e) supplyingthe expanded fluid to a separator vessel having a top and a bottom; (f)withdrawing from the bottom of the separator vessel a liquid productstream; (g) withdrawing from the top of the separator vessel a gaseousstream; and (h) introducing the gaseous stream obtained in step (g) asfeed and/or fuel in the process for the preparation of liquidhydrocarbons, which process for the preparation of hydrocarbonscomprises converting a light hydrocarbonaceous feedstock into synthesisgas, followed by catalytic conversion of the synthesis gas into liquidhydrocarbons, and in which process the gaseous stream which is used instep (h) is used in the production of hydrogen to be used in theproduction of liquid hydrocarbons and in which the hydrogen is purifiedby means of PSA.
 2. The process of claim 1, in which the gaseous streamis used as feed and/or as fuel in the production of hydrogen by means ofsteam methane reforming.
 3. The process of claim 1, in which the gaseousstream which is used in step (h) is used in the production of synthesisgas to be used in the production of liquid hydrocarbons.
 4. The processof claim 3, in which the synthesis gas is made by partial oxidation. 5.The process of claim 3, in which the synthesis gas is made by steammethane reforming, the gaseous stream being the feed for the reforming,or in which the synthesis gas is made by a combination of steam methanereforming and partial oxidation.
 6. The process of claim 1, in which theseparator vessel is provided with internals in order to improve theseparation of low boiling point components and the liquid productsstream.
 7. The process of claim 1, in which any off gas from thehydrocarbon synthesis is used for the production of power to be used inthe liquefaction process.
 8. The process of claim 1, in which surplusenergy which is produced in the partial oxidation process and/or thehydrocarbon synthesis process is used for the production of power to beused in the liquefaction process.